ISO Week Numbers 2021
2021 has 52 ISO 8601 weeks. Week 1 runs from 4 Jan to 10 Jan, and week 52 ends on 2 Jan 2022. 1 January 2021 sits in week 53 of 2020 and 31 December 2021 in week 52 of 2021.
| Week | Monday | Sunday | Month | Falls in this week |
|---|---|---|---|---|
| 1 | 4 Jan | 10 Jan | January | |
| 2 | 11 Jan | 17 Jan | January | |
| 3 | 18 Jan | 24 Jan | January | |
| 4 | 25 Jan | 31 Jan | January | |
| 5 | 1 Feb | 7 Feb | February | Groundhog Day |
| 6 | 8 Feb | 14 Feb | February | Valentine's Day |
| 7 | 15 Feb | 21 Feb | February | Mardi Gras |
| 8 | 22 Feb | 28 Feb | February | |
| 9 | 1 Mar | 7 Mar | March | Spring |
| 10 | 8 Mar | 14 Mar | March | Pi Day, International Women's Day |
| 11 | 15 Mar | 21 Mar | March | St. Patrick's Day, the Spring Equinox, the International Day of Happiness |
| 12 | 22 Mar | 28 Mar | March | Earth Hour, World Water Day |
| 13 | 29 Mar | 4 Apr | April | Easter, April Fools' Day, Good Friday |
| 14 | 5 Apr | 11 Apr | April | World Health Day, National Pet Day |
| 15 | 12 Apr | 18 Apr | April | Tax Day |
| 16 | 19 Apr | 25 Apr | April | Earth Day, World Book Day |
| 17 | 26 Apr | 2 May | April | May Day |
| 18 | 3 May | 9 May | May | Mother's Day, Cinco de Mayo, Star Wars Day |
| 19 | 10 May | 16 May | May | |
| 20 | 17 May | 23 May | May | |
| 21 | 24 May | 30 May | May | |
| 22 | 31 May | 6 Jun | June | Summer, Memorial Day, World Environment Day |
| 23 | 7 Jun | 13 Jun | June | World Oceans Day |
| 24 | 14 Jun | 20 Jun | June | Father's Day, Juneteenth |
| 25 | 21 Jun | 27 Jun | June | the Summer Solstice |
| 26 | 28 Jun | 4 Jul | July | Independence Day |
| 27 | 5 Jul | 11 Jul | July | |
| 28 | 12 Jul | 18 Jul | July | World Emoji Day |
| 29 | 19 Jul | 25 Jul | July | |
| 30 | 26 Jul | 1 Aug | July | |
| 31 | 2 Aug | 8 Aug | August | |
| 32 | 9 Aug | 15 Aug | August | Back to School |
| 33 | 16 Aug | 22 Aug | August | World Photography Day |
| 34 | 23 Aug | 29 Aug | August | National Dog Day |
| 35 | 30 Aug | 5 Sep | September | Autumn |
| 36 | 6 Sep | 12 Sep | September | Labor Day |
| 37 | 13 Sep | 19 Sep | September | Talk Like a Pirate Day |
| 38 | 20 Sep | 26 Sep | September | the Autumn Equinox, the International Day of Peace |
| 39 | 27 Sep | 3 Oct | September | International Coffee Day, National Coffee Day |
| 40 | 4 Oct | 10 Oct | October | World Teachers' Day, World Mental Health Day |
| 41 | 11 Oct | 17 Oct | October | Columbus Day, World Food Day |
| 42 | 18 Oct | 24 Oct | October | |
| 43 | 25 Oct | 31 Oct | October | Halloween, National Cat Day |
| 44 | 1 Nov | 7 Nov | November | |
| 45 | 8 Nov | 14 Nov | November | Veterans Day, Singles' Day, World Kindness Day |
| 46 | 15 Nov | 21 Nov | November | International Men's Day, World Television Day |
| 47 | 22 Nov | 28 Nov | November | Thanksgiving, Black Friday |
| 48 | 29 Nov | 5 Dec | December | Cyber Monday, Winter, Giving Tuesday |
| 49 | 6 Dec | 12 Dec | December | Human Rights Day |
| 50 | 13 Dec | 19 Dec | December | |
| 51 | 20 Dec | 26 Dec | December | Christmas, Boxing Day, the Winter Solstice |
| 52 | 27 Dec | 2 Jan 2022 | December | New Year's Eve |
Why 2021 has 52 weeks
An ISO week year has 53 weeks when 1 January falls on a Thursday, or when the year is a leap year whose 1 January falls on a Wednesday. Every other year has 52. Because 52 weeks is only 364 days, the calendar has to absorb the leftover day or two somewhere, and a 53-week year is where it does that. Payroll systems, retail reporting and production planning all trip over this, because a 53-week year has one extra reporting period.
The year boundary
The first and last weeks of an ISO year rarely line up with the calendar year. Week 1 of 2021 starts on 4 Jan, and 1 January 2021 belongs to week 53 of 2020. This is not an error in the calendar: it follows directly from the rule that week 1 is the week containing the first Thursday, which keeps every week whole and every week in exactly one year.
Other years
The same calendar for 2020 to 2035. Years with 53 weeks are marked.
Frequently asked questions
How many weeks are in 2021?
2021 has 52 ISO 8601 weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday; otherwise it has 52.
Which week is 1 January 2021 in?
1 January 2021 falls in ISO week 53 of 2020. That is the previous ISO week year, because week 1 is the week containing the first Thursday, and in 2021 that week starts later.
Which week is 31 December 2021 in?
31 December 2021 falls in ISO week 52 of 2021.
When does an ISO week start?
On Monday. ISO 8601 defines the week as running Monday to Sunday, and week 1 as the week containing the first Thursday of the year, which is the same as the week containing 4 January. That is why an ISO week calendar can open in late December.
Need the week number for one specific date instead? Use the week number tool. Want this calendar as data? It is published as free JSON at /api/weeks/2021.json, documented on the data page.
Computed from the ISO 8601 rule, not from a stored table. Last updated 2026-08-05.
Related tools
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